Attenuation circuit



June 6, 1961 M. L. MILLER ErAL ATTENUATION CIRCUI'T Original Filed Feb.18, 1957 MMM Haro/a/Ja/maf?, A

United States Patent 9C) 2,987,678 AT'IENUATION CIRCUIT g Meritt L.Miller and Harold J. Dalmau, Fort Wayne,

Ind., assignors to General Electric Company, a corporation 'of New YorkContinuation of application Ser. No. 640,672, Feb. 13, 1957. Thisapplication Nov. 13, 1959, Ser. No. 852,901 3 Claims. (Cl. 33o-109)'I'his invention relates to transmission circuits, and more particularlyto attenuation circuits having improved band pass characteristics so asto provide substantially flat re spouse to frequencies throughout anextended band except at the selected attenuated frequency. Thisapplication is a continuation of the copending application of Mer-itt L.Miller and Harold J. Dalman, Serial No. 640,672, filed February 18, 1957for attenuation circuit, and now abandoned.

'Ihere are many transmission circuits where the attenuation of aparticular frequency is desired with as little eiect on the otherfrequencies as possible. While it has not been diflicult to obtainsubstantially complete elimination of the selected frequency, previouslyused circuits have tended to have an attenuation effect which increasedgradually up to the selected frequency and which therefore had asubstantial effect on frequencies adjoining it. It is most desirable toprovide an attenuation circuit wherein the attenuation effect at theselected frequency y, 2,981,618 Patented June l6, 1961 FIGURE 2 is aschematic circuit diagram of a preferred embodiment of the invention;and

FIGURE 3 is a graph illustrating the operating characteristics of thecircuit of the invention.

Referring now to the figures of the drawing, there is shown an amplifier1 having an input terminal 2 and an output terminal 3; amplifier 1 maybe conveniently grounded as shown at 4. Considerable discretion may beutilized in the selection of the particular amplier; howwill remainunimpaired but almost no attenuation will v occur in the frequency bandon either side of the selected frequency.

It is therefore an object of this invention to provide an attenuationcircuit which will have an improved band pass characteristic with asubstantially at response to all frequencies except the one selectedfrequency, at which the circuit will exhibit a sharp attenuationcharacteristic.

A further object of the invention is to provide an ampliiier and aparallel T network in a regenerative feedback circuit which will provideconsidera-bly greater selectivity at the desired frequency than isnormally obtained with a parallel T network.

In one aspect thereof, the invention provides an attenuation circuitwhich includes an amplifier. A parallel T network includes a pair ofVresistances in series, a pair of capacitances in series connected inparallel with the resistances, a third resistance connected between thejunction of the pair of capacitances and the amplifier output terminal,and a third capacitance connected between the junction of the pair ofresistances and the output terminal. The pairs of resistances andcapacitances are connected at one end to the amplifier input terminal.With this connection, the other end of the pairs of resistances andcapacitances is adapted to be connected to receive an input signal.Provided the amplifier has a 10W output impedance relative to theimpedance of the parallel T network, the regenerative feedback circuiteffect which is obtained by connecting the network to a point in theamplilier having a gain of no more than unity causes the arrangement tohave considerably greater selectivity in attenuating a single frequencyand in leaving the other frequencies substantially unaffected.

In the drawing:

FIGURE 1 is a block diagram of the invention;

ever, as will appear below, care must be taken in the selection of afeedback point in the amplifier to ensure that the gain at that point beno greater than unity otherwise the stability of the circuit isimpaired. Also, the signal at the point must be less than out of phasewith the input signal to the amplifier to ensure the desiredregenerative effect. 'Ihe output impedance of amplifier 1 must be lowrelative to the impedance of the filter network which is to bedescribed. Preferably, the proportion of amplifier output impedance tonetwork impedance should not exceed about ten percent, although thisfigure should not be taken to represent a critical limitation.

A parallel T network, generally indicated at 5, is provided. As iscustomary, network 5 includes a pair of resistances 6 and 7 in series,and a pair of capacitances 8 and 9 which are in parallel with theresistances and in series with each other. A third resistance 10 isconnected to a junction point 11 between the capacitances y8 and 9 and athird capacitance 12 is connected to a junction point 13 between theresistances 6 and 7. It will, of course, be understood that suitableselection of the resistances 6, 7 and 10 and of the capacitances 8, 9and 12 determines the selection of the frequency which is to beattenuated, and the degree of symmetry of the band pass characteristics.A suitable input signal tobe corrected is applied at point 14 at one endof the parallel pairs of impedances; the other end of the pairs ofimpedances is directly connected to the input terminal 2 of amplifier 1so that the amplifier in effect receives the output of the filternetwork 5.

A regenerative feedback circuit is obtained by connecting the other endsof resistance 10 and capacitance 12 to a suitable point 32 in theamplifier through an appropriate conductor 15. The point must be onewhere the gain is unity or less (even if the amplifier, considered as awhole is high gain), and the phase must not -be more than 90 degreesfrom the phase of the input signal in order to be sure of providing atrue regenerative effect, i.e., increasing the amplification. Atfrequencies which are removed from the selected frequency, theregenerative circuit has the effect of tending to maintain the measuredvoltage across the network at a level substantially unimpaired by thenormal attenuation function of this type of network. However, at theselected frequency, the filter network overcomes the regenerative effectto perform its normal attenuating function, that is, the network behavesin substantially the same manner as if its impedances 10 and 12 weregrounded instead of being connected into the amplifier 1. Thus, at theselected frequency, filter network 5 will perform the desiredattenuating function while at all other frequencies, including thoserelatively close to the selected frequency, the regenerative feedbackeffect will maintain the signal substantially unattenuated.

The response obtained with the connections of FIG- 3 URE 1 isillustrated by curve A of FIGURE 3, while the response obtained by thenormal parallel T connection, wherein impedances 12 and 10 are connectedto ground, is indicated by curve B. These curves will be more fullydiscussed in connection with the specific embodiment of FIGURE 2.

Referring now to FIGURE 2, it will be observed that it includes filternetwork the same numerals are used for the parts of this network as inFIGURE 1. Network 5 is connected at point 2 to the grid 17 of a triodevacuum tube 18 so that the output signal from the network 5 is(impressed across the grid 17. Tube 18 also includes a plate 19 and acathode 20 with a load resistor 21 connected between the cathode andground as indicated at 22. This type of circuit, including tube 18 andresistor 21, s commonly called a cathode follower circuit and ischaracterized both by the fact that the gain can never enceed unity andthat for a given potential the cathode 20 will follow grid 17 so as tomaintain a predetermined potential between them.

The necessary direct current plate potential is impressed on plate 19 byconnecting it to a source 23 of direct current power which in turn isconnected to ground as shown at 24. -In order to raise the steady statecurrent in the circuitry of FIGURE 2 to a high enough value tofacilitate operation, a resistance 25 may be connected between the plate19 and the input point 14 of network S, as shown, and a secondresistance 26 which is connected to ground as shown at 27, may beprovided to complete this connection. In order to preclude Ithe directcurrent plate supply from being impressed outside the filtering circuitconstituting the invention, a pair of blocking capacitors 28 and 29 maybe provided. `Capacitor 28 is provided in line 30 through which theinput signal passes to point 14, and capacitor 29 is provided in theoutput line 31 through which the output signal from the cathode followerpasses.

As stated before, network 5 receives an input signal to be filtered atpoint 14, and from point 2 has its output impressed across grid 17. Theregenerative feedback circuit broadly described in connection withA FIG-URE 1 is achieved by connecting impedances 10 and 12 by a line 15 to theoutput line 31. This is feasible, in this particular case, because thegain, even at the output terminal, is less than unity with theparticular circuit described. Also, the phase is necessarily the same asthat of the input signal in this case, which represents the optimumphase relationship yfor regenerative purposes.v The connection describedfeeds the output signal back into the network so as to opposeattenuation of the signal until the selected frequency is reached.

A network 5 having the following resistances and capacitancesrespectively was tested in a circuit of the type shown in FIGURE 2:

Resistance 6 ohms-- 10,000 Resistance 7 do 100,000 Resistance do 10,000Cacapitance 8 microfarads-- f1.0 Capacitance 9 do 0.1 Capacitance 12..-do 1.0

The vacuum tube was a simple triode of the 58'14 type, and resistor 21was provided with a value of 15,000 ohms. The output impedance of thecathode follower circuit was 200 ohms, 0r 2% of the network impedance.With an ordinary parallel T network connection having the aboveresistance and capacitance values, it was` found that the attenuation indecibels with respectV to the frequency varied as shown by curve B ofFIGURE 3. This curve shows that at an entire octave away from theselected frequency, the filter network has an attennation of tendecibels andv that at half an octave the attenuation has risen to 17decibels; This degree of attenuation so far away from the' selectedfrequency repfilter network. However, when the apparatus was connectedas described in FIGURE 2, a curve as shown by A in FIGURE 3 wasobtained. The attenuation at a full octave from the selected frequencyhad been cut to less than half a decibel (which represents only about4%) and at half an octave away the loss is still less than one decibel,which represents less than 9 percent. It is thus seen that theregenerative feedback action achieved by the circuitry of the inventionprovides a material advantage in improving the band pass characteristicsof the parallel T filter network Without any material effect on theattenuation at the selected frequency.

While the invention has been explained by describing a particularembodiment thereof, it will be apparent, particularly in the light ofFIGURE 1, that improvements and modifications may be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An attenuation circut comprising an amplifier having an inputterminal and a signal output circuit, and a parallel T network includinga pair of resistances in series, a pair of capacitances in seriesconnected in parallel with said resistances, a third resistance havingone end connected to the junction of said pair of capacitances and athird capacitance having one end connected to the junction of said pairof resistances, said third resistance and capacitance having their otherends connected solely to a point on said amplifier output circuit forfeeding back at least a portion of the output signal of said amplifierto said T network, said pair of resistances and said pair ofcapacitance's being connected at one end to said amplifier inputterminal and being adapted to be connected at their other end to receivean input signal, said amplifier having a low output impedance relativeto the impedance of said network, the gain resentsY a serious limitationon the use of aparallel T at said point on said amplifier output circuitbeing no more than unity and the phase at said point being less thanremoved from the phase Yat said input terminal, said gain and phasebeing continuously maintained during operation of the circuit and saidcircuit having an attenuation at one-half octave from the selectedfrequency of no more than one decibel, said attenuation at the selectedfrequency tending to approach iniinity.

2. An attenuation circuit comprising an amplifier having an inputterminal and a signal output circuit, and a parallel T network includinga pair of resistances in series, a pair of capacitances in seriesconnected in parallel with said resistance, a third resistance havingone end connected to the junction of said pair of capacitances and athird capacitance having one end connected to the junction of said pairof resistances, said third resistance and capacitance having their otherends connected solely to a point on said amplifier output circuit forfeeding back at least a portion of the output signal of said amplifier-to said T network, said pair of resistances and said pair ofcapacitances being connected at one end to said amplier input terminaland being adapted to be connected at their other end toreceive an inputsignal, said amplifier having a low output impedance relative to theimpedance of said network, the gain at said point on said amplifieroutput circuit being no more than unity and the phases at said point andat said input terminal being substantially thesame, said gain and phasebeing continuously maintained during operation of the circuit and saidcircuit having an attenuation at one-'half octave from the selectedfrequency of no more than one decibel, said attenuation at the selectedfrequency tending to approachr infinity.

3. An attenuation circuit comprising a cathode follower circuitincluding a vacuum tube having at least an anode, a cathode and a grid,said cathode follower circuit further including a load resistanceconnected between Said cathode and ground, and a parallel T networkincluding a pair of resistances in series, a pair of capacitances inseries connected in parallel with said resistances, a third networkresistance connected between the junction of said pair of capacitancesand the junction of said load resistance with said cathode, a thirdcapacitance connected between the junction of said pair of resistancesand the junction of said load resistance with said cathode, said pair ofresistances and said pair of capacitances being connected at one end tosaid grid and being adapted to be connected at their other end toreceive an input signal, said amplifier having a low out- 5 ing toapproach infinity.

References Cited in the file of this patent UNITED STATES PATENTSLavender Sept. 3, 1946 Rieke a Jan. 11, 1949

